In optical communication, it is demanded to transmit a large amount of data over a long distance. As a method for transmitting light over a long distance, a subcarrier multiplexing transmission system is known. Further, a method for raising the frequency utilization efficiency in order to transmit a large amount of data is known. For example, as a method for raising the frequency utilization efficiency, a hybrid transmission method that uses a plurality of modulation formats is known.
Also a frequency division duplex technology or a like technology is known in which different modulation formats (hybrid modulation) are used for different subcarriers of a subcarrier multiplexing transmission system and frequencies different between transmission and reception apparatuses are allocated to perform full-duplex communication.
Data of transmission distances in the modulation formats of, for example, quadrature phase shift keying (QPSK), 8 quadrature-amplitude-modulation (QAM) and 16 QAM are known. It is known that the transmission capacity is increased by changing the modulation format between QPSK and 8 QAM or 16 QAM according to the transmission distance (refer, for example, to X. Zhou et al., “High Spectral Efficiency 400 Gb/s Transmission Using PDM Time-Domain Hybrid 32-64 QAM and Training-Assisted Carrier Recovery,” JOURNAL OF LIGHTWAVE TECHNOLOGY, VOL. 31, NO. 7, April, 2013).
Data in which an efficiency when data are transmitted using a single subcarrier having a great transmission capacity and an efficiency when data are transmitted using a plurality of subcarriers each having a small transmission capacity are compared with each other are known (refer, for example, to T. Oyama, H. Nakashima, T. Hoshida, T. Tanimura, Y. Akiyama, Z. Tao and J. C. Rasmussen, “Complexity Reduction of Perturbation based Nonlinear Compensator by Sub band Processing,” OFC, paper Th3D. 7, 2015).